A 45-year-old man on intensive phase anti-TB therapy complains of difficulty distinguishing traffic light colours after 3 months of treatment. Visual acuity testing shows bilateral reduction and loss of red-green colour discrimination. The most likely causative drug and its dose-dependent toxic target are:
- A Isoniazid, optic nerve demyelination
- B Streptomycin, vestibulocochlear nerve damage
- C Ethambutol, retrobulbar neuritis affecting the papillomacular bundle ✓
- D Rifampicin, cortical visual processing defect
Correct answer: C. Ethambutol, retrobulbar neuritis affecting the papillomacular bundle
Explanation
Ethambutol causes a dose-dependent optic neuropathy that classically presents with impaired red-green colour vision before loss of visual acuity, reflecting damage to the papillomacular bundle. The risk rises above 15 mg/kg/day and in renal impairment. Isoniazid can rarely cause optic neuritis, but colour vision loss as the earliest sign points to ethambutol. Baseline and monthly visual assessment is advised.
Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.